Chromium plating



Patented Aug. 17, 1954 UNITED STATES PATENT OFFICE CHROMIUM PLATING Jesse E. Stareck, Royal Oaks, and Ronald Dow, Detroit, Mich., assignors to United Chromium, Incorporated, New York, N. Y., a corporation of Delaware Application May 20, 1953, Serial No. 356,188

7 Claims. 1

This invention relates to chromium plating using a chromic acid plating bath having a selfregulated catalyst acid radical content. The invention particularly relates to a method for producing crack-free deposits of chromium.

In co-pending application, Serial No. 194,502, filed November 7, 1950, now Patent No. 2,640,022,

upon which the present application is an imbeen discovered that crack-free chromium plate can be deposited by maintaining, over a range of chromic acid concentration, a particular and well defined range of concentration of the catalyst acid radicals, as hereinafter described, and

by maintaining the temperature of the bath 5.

solution above a particular minimum value. The surprising result of a crack-free plate is additional to the advantages above referred to Which are described in detail in the said copending application. Not only is the present plate crackfree, but also it is essentially pore-free.

The present method is applicable to general chromium plating, such as described in said copending application. It is particularly applicable to the plating of metals that are subject to -]corrosion, such as steel, and has special applil cation to the deposition of decorative plate which heretofore required corrosion resistant undercoats of nickel, or copper, or nickel and copper, etc. As is known, attempts to deposit decorative chromium plate having the usual thickness of this type of plate in the absence of a protective undercoat results in a porous, noncorrosion resistant plate which does not cover the pores in the surface of the basis metal; on the other hand, the deposition directly on the basis metal of chromium plate of greater thickness results in a cracked plate which is not corrosion resistant. In conventional decorative plating, for example in the case of a conventional automotive steel bumper, the bumper may 1 be plated with 2mils of nickel and 0.01 mil of chromium; or with l mil of copper, 1 mil of nickel, and 0.01 mil of chromium; or with 1 mil ofcopper, 2.0 mils of nickel and 0.01 mil of the decorative plate range, viz. 0.01 to 0.02 mil. 6

By plating about 0.2 to about 2.0 mils of crackfree chromium directly on the bumper according to the present method, a product comparable to conventional bumpers in appearance and in resistance to corrosion may be produced, but by eliminating the necessity for nickel, copper, nickel and copper, or other undercoats, an important simplification of the plating cycle and economies in time, equipment, and labor costs are obtained. Not only automotive parts but others, such as parts for electrical and mechanical appliances, and, in general, parts requiring a decorative, corrosion-resistant deposit, may be plated according to the invention at substantial savings and with greater convenience. The thickness of the present plate may be approximately equal to the combined thicknesses of the nickel chromium, copper-chromium, coppernickel-chromium, etc. deposits on conventional articles; and in some cases the thickness may be reduced over that of conventional articles, affording a saving in weight as well as in material. With the present chromium plate as the coating, a harder finish is provided which is desirable for many kinds of articles, and the plate can be buffed to a bright appearance where brightness is required. It will also be apparent that the present plate can be deposited in place of conventional chromium plate, that is, over conventional undercoats, and in such cases a corrosionresistant product is obtained which is superior to the conventional product and whose total deposit (undercoats plus chromium) as thinner.

Crack-free plate, particu arly heavy plate, but including other kinds as well, can be produced on parts and articles useful in engineering applications such as on chemical and manufacturing equipment, engine and machine parts, and the like, in order to meet unusual problems of corrosion and erosion, wear and abrasion, temperature efiects, etc.

The'bath solution used in the method comprises chromic acid, two catalyst-supplying compounds, namely, a sulfate radical bearing salt and a. silicofiuoride radical bearing salt, each having a limited solubility in the chromic acid bath, and two soluble non-catalytic compounds one of which is a, compound of the cation. associated with the sulfate salt and the other of which is a compound of the cation associated with the silicofluoride salt. The non-catalytic compounds have the effect of controlling the concentrations of the dissolved sulfate and silicofluoride radicals in the bath by influencing the solubility of the salts used to introduce the radicals into the bath. The specific catalyst-supplying compounds are strontium sulfate and an alkali metal silicoiluoride, the alkali metal being selected from the class consisting of potassium and sodium. Specific non-catalytic compoiuids are strontium carbonate, strontium oxide, strontium chromate, strontium hydroxide, potassium hydroxide, potassium bichromate, potassium carbonate, potassium chromate, sodium. bichromate, sodium carbonate, sodium hydroxide, sodium chromate.

The concentration of the chromic acid, expressed as CrOe, in the bath is about 200 to 900 g./l. The amount of the strontium sulfate and of the alkali metal silicofluoride, is, in each case, sufficient to saturate the bath with dissolved sulfate and silicofluoride and to provide an insoluble residue of each salt in the bath. The soluble non-catalytic strontium and alkali metal compounds are each present in an amount sufiicient to adjust or suppress the concentration of the strontium sulfate and alkali metal silicofiuoride, respectively, in solution in the bath from the unsuppressed saturation concentration of the latter two compounds to a lower concentration equivalent to 1.5 to 11.7 g./l. of the sum of the sulfate (SO4=) and silicofluoride (SiFF). The concentration of dissolved sulfate may vary from 0.3 to 5.5 g./l. and that of dissolved silicofluoride from 1.0 to 11.4 g./l., with the sum of the two catalysts lying in the range of 1.5 to 11.7 g./l. H

The sum of the dissolved sulfate and silicofiuoride required varies with the dissolved CIOs in the manner defined by the area ABCD of the graph shown in the accompanying drawing. As noted in the drawing, the graph shows the variation of the chromic acid concentration in grams per liter with the concentration, in grams per liter, of the sum of dissolved sulfate and silicofiuoride. As the chromic acid varies from 200 to 900 g./l.,

the required catalyst content varies from a low of 1.5 g./l. to a high of 11.7 g./l. Within the area ABCD,'the plate that is deposited is crack-free and has the other advantages described hereinafter.

lyst concentrations below the curve AB, the plate is nodular and unsuitable. The above-described variation of the chromic acid with the sum of the sulfate and silicofluoride may also be shown by Table 1, which also shows how the sulfate and Preferably, the amounts of the non-catalytic strontium and alkali metal compounds are such as to suppress the concentration of the strontium sulfate and alkali metal silicofiuoride, respectively, in solution in the bath from the unsuppressed D At catalyst concentrations that lie above the curve DC, the plate is cracked, and at catasaturation concentration of the latter two compounds to a lower concentration equivalent to 1.9 to 7.6 g./l. of the sum of the dissolved sulfate and silicofluoride. The preferred concentration of dissolved sulfate may vary 0.3 to 4.5 g./l. and that of dissolved silicofiuoride from 1.5 to 6.3 g./l. with the preferred sum of the two catalysts lying in the range of 1.9 to 7.6 g./l. The preferred variation of the sum of the dissolved catalyst radicals with the CrOe concentration, therefore, is as defined by the area JKLM of the graph, and is also set forth in the following Table 2:

Table 2 CrO; SOAI'SiFfl SO; SiFs g./l. ./Z. g./l. g./l.

It is desirable that the concentration of sulfate should exceed that of silicofiuoride in the area EBF of the graph and that the concentra- Sum of S04 and SiFa CrOa, g./l. Range in Range in which $04 which Sil a shoulgl exceed should exceed F5 so.

The chromic acid content may be supplied by adding chromic acid as such to the bath, although chromate, sodium bichromate, strontium chromate, etc. It will be understood that in referring to the bath chromic acid content, it is intended to include, unless specified otherwise, the ClOa added per se and any CIO3 added in the form of one of the foregoing non-catalytic compounds.

The plating bath may be made up by adding a mixture of the solid ingredients, in subdivided form, to water and stirring until equilibrium is obtained. The bath may also be maintained in thisway.

The plating method comprises passing current in the range of 0.5 to 8, preferably 1 to 3, amperes per square inch (A. S. I.) from an anode to an article cathode immersed in the bath. The bath is operated at a minimum temperature of F., it having been found that below this temperature the plate that is produced is cracked. The upper temperature may extend to the boiling point of the bath but prefhigher, and higher current densities are used.

The crack-free quality of the chromium plate produced by the present method is not only determinable visually but also by means of other tests, one of which is the Dubpernell test for detecting cracks in chromium plate as described in Baker et al., S. A. E. Journal, 22pages 321-334, note page 334 (March 1928) Baker et al., Trans. Am. Electro-Chem. Soc., 54 pages 337-346, note page 338 (September 22, 1928). Essentially, in this test, acid copper is plated on the chromium plated article, and if there are any cracks in the chromium plate, the copper will deposit preferentially in them. This test showed the present plate to be crack-free. Another test is microscopic examination, using from to 200 power magnification. No cracks were found in the present plate by microscopic examination. Another test is the anodic etch test in which, with the chromium plated article as the anode, chromium is dissolved away from the article; if cracks are present or incipient, they are developed during this test and become readily visible. Even on anodic etching, the present plate showed no cracks.

The crack-free plate of the invention is also characterized by having a hardness of about 425 to about 825, usually 550 to 700, Knoop. The plate is relatively soft and ductile as compared with conventional chromium plate. It has a smooth texture, a satiny finish, and a dull, matte, white color. It is easy to polish or bufi.

The invention may be illustrated by the following examples:

EXAMPLE 1 A solid, finely divided mixture was made up consisting of 50% CrOs, 26.25% potassium dichromate, 5.25% strontium chromate, 12.5% potassium silicofiuoride, and 6.0% strontium sulfate, these percentages being on a weight basis. An aqueous solution was prepared containing 390 g./l. of this mixture, and after reaching equilibrium, the solution contained 275 g./l. dissolved CrOs, 0.8 g./l. dissolved sulfate 80F and 2.0 g./l. dissolved silicofiuoride SiFs=. The ClOs content was equivalent to the'total hexavalent chromium in solution. A mild or low carbon steel rod was plated in this solution at 140 F. for 2 hours using 3 A. S. I. In order to determine whether a crack pattern was present in the deposit, anodic etching was carried out by making the rod anode in the same solution and passing 2.5 A. S. I. for 10 minutes; inspection of the chromium plate with a 10-power microscope showed that it was crack-free.

EXAMPLE 2 A mixture was prepared consisting of, on a weight basis, 34.4% CrOs, 61.9% sodium dichromate, 2.0% strontium chromate, 1.1% sodium silicofiuoride, and 0.6% strontium sulfate. The

After reaching 7 i 6 0.2 to 0.3 mil, and both had a satiny finish. One of the articles was buffed to a bright luster to demonstrate that the appearance of the plate could be substantially improved by a simple buff- 1 ing operation.

EXAMPLE 3 The following mixture, consisting of 72% CIOs, 21% potassium dichromate, 5.0 strontium chromate, 1.3% potassium silicofiuoride, and 0.7% strontium sulfate, weight basis, was added to water to provide a solution containing 356 g./l. of the mixture, and after equilibrium the solution contained 310 g./l. dissolved CrOs, 0.8 g./l. dissolved sulfate SO4=, and 2.0 g;/l. dissolved silicofiuoride SiF6 Using a temperature of 140 F. and a cathode current density of 2 A. S. I., steel rods, 3" lon x diameter, were plated in this solution for 2 hours, a satisfactory crack-free chromium plate of 2.1 mils thickness being obtained.

EXAMPLE 4 A sheet steel panel, 3" x 4", identified as No. l, was plated in a commercial chromium plating hath made from chromic acid, potassium silicofiuoride, strontium sulfate, potassium dichromate, and strontium chromate, and having 204.0 g./l., of dissolved CIOs, 12.8 g./l. of dissolved potassium silicofiuoride, and 6.2 g./l. of dissolved strontium chromate; the bath temperature was 120 F. and the current density 2 A. S. I. A thickness of approximately 0.3 mil of chromium was plated on the panel. At the same time three sheet steel panels (Nos. 2, 3 and 4) of the same kind and size as panel No. 1 were plated according to the method of Example 2. The thickness of the chromium plate was approximately 0.3 mil on panels Nos. 2 and 3 and 0.3 to 0.4 mil 1 the spray. After 24 hours exposure, Nos. 1 and 2 were removed from the salt spray and examined. The chromium plate on the exposed side of No. l had completely failed to survive the attack of the salt spray; on approximately half the area of the exposed side the underlying basis metal was visible, while the other half of the area comprised rust distributed over the entire side. The exposed side of panel No. 2 showed no sign of attack. After hours, panel No. 3 was removed and showed rusting along its edges and at intermediate isolated spots; less than about 10% of the area of the exposed side of No. 3 had been attacked. After 175 hours, panel No. 4 was removed and examined; rusting was apparent along most of its edge portions and in some intermediate areas, about 10 to 20% of the area of the exposed side having been attacked. A panel plated at F. and 2. A. S. I. in a conventional bath made from chromic acid and sulfuric acid and having 250 g./l.. of dissolved C103 and 2.5 g./1. of dissolved sulfate showed results similar to panel No. 1.

EXAMPLE 5 Fifteen sheet steel panels, identified by the letters A to 0, were plated in various ways, described below, and subjected to outdoor exposure tests. Following the exposure, the ap pearance and condition of the exposed side of each panel was noted and is briefly described in the following table.

Panels A to F were chromium plated directly, panels A and D in the same commercial chro- 7" mium plating'batht and using, the same bath c n ditions as. described in Example 4-, and: panels B; C; E: and E according to the method of Ex ample, 2;. Panels 6- and. F were buffed following the plating. The six panels were subjected to outdoor exposure for; 142 days in Detroit, the time of the exposure being from January to May, a time of the year when corrosion is most severe.

A second group. of panels identified as G, H and I were, conventionally plated with undercoatsbefore being chromium plated by the method of Example 2. Panel G was plated first with copper, then with nickel, and then with chromium; panel H was plated with nickel, then chromium; and panel I With copper followed by chromium. This group of panels was exposed out-of-doors in Detroit during January to June for. 177 d ys. v

Panels J K and L were conventionally plated with copper and then with conventional chromium plate, using the conventional bath and conditions described at the end of Example 4, While panels. M, N and 0 were conventionally plated with copper. then with nickel, and then with the conventional chromium plate. Panels J. to 0- were all. exposed out-of-doors in Detroit during January to, June for 1 77 days.

With regard to the description in Table 4, in which pitting of the plate is described, a small number of pits' is indicated by the expression sparsely scattered, and progressively larger numbers are indicated by the expressions scattered, numerous? very numerous, heavy and very' heavy. The commercial and conventional' plates are designated as prior plate.

Table 4 8. EXAMPLE 6 From a mixture of 54.7% chromic acid, 333% potassium dichromate, 8.6% strontium chromate, 2.5% potassium silicofluoride, and 0.9% strontium sulfate, weight basis, a bath solution was made up containing 270 grams of the mixture per liter of solution. The solution analyzed 220 g./l. of dissolved CrO3, 1.6 g./l. of dissolved silicofluoride, and 0.4 g./l. of dissolved sulfate. A steel rod 3" long x diameter was plated in this solution at 150 F. and 3 A. S. I. for 3 hours. The plate produced was crack-free.

EXAMPLE 7 A mixture comprising 54.7% chromic acid, 33.2% potassium dichromate, 8.6% strontium chromate, 2.5% potassium silicofiuoride, and 0.9% strontium sulfate, all percentages being by weight, was prepared and added to water to pro! vide a solution having 200 g./l. of dissolved CrOa. 2.8 g./l. of dissolved silicofluoride, and 0.5 g./l-. of dissolved sulfate. A steel mandrel, 4- long x diameter, was plated in this solution at 6 A. S. I. for one hour at 160 F., a crack-free chromium deposit being produced that was dull and smooth. The thickness of the deposit was 2.5 mils. The plated mandrel was then anodical. 1y treated in the same solution at 1 A. S. I. for 5 minutes to develop any crack structure that might be present, but no cracks were found. Another mandrel of the same size as the preceding one was then plated in the same bath solution at 8 A. S. I. for 0.5 hour at 180 F., there being produced on the mandrel a crack-free deposit of 35 chromium that was dull and smooth and had a Chromium plate Undercmt' thickness, mils Outdoor Panel. 7 r V expo- Thick- P C M f Remarks Metal: mess rior rae ree ays mils Plate ate A 0.2 142 Entire area heavily rusted Fr, 0.5 buffed. 142

copper. 0. 5 {nickel 0. 5 177 H- nickel... 1.0 0.05 177 I COPPER. 1.0 0.05 l. 177 J .060..." 1 0 0.05 177 {err-.1 8:2,} or {asst-.1 8:2} m

size.

Like G, but color lighter.

Except along one edge portion, entire area quite dark and heavily pitted; cracks visible throughout.

Like J, but less cracks visible.

Like J, but no cracks visible.

{Very numerous pits through,-

out entire area with edge portions noticeably darkened.

Numerous pits throughout area with network of fine cracks in entire area.

Substantially like M;

Whether plated directly on the basis metal or over undercoats, it will be seen from Example 5 that the crack-free plate, of the invention is superior to the prior plate.

thickness of 1.06 mils. This mandrel was also anodically treated in the same, solution using 1, A. S. I. for 2.5 minutes, but no crack, structure was apparent.

In the examples and elsewhere the weight percentages of the ingredientsof the solid mixture for making up the bath solution can also be expressed as parts by weight.

Deposits of varying thicknesses may be plated by the present method, ranging for example from a thickness just sufiicient to cover the pores of the basis metal to any practical desired thicker deposit for which a demand may exist.

The invention is useful for articles made of any of a variety of basis metals, such as plain carbon steels, alloy steels including stainless steel, iron, cast iron, copper and copper alloys, nickel and nickel alloys, zinc and zinc alloys, alumium and alumium alloys, tin and tin alloys, lead and lead alloys, etc. In general, the basis metal may be any metal, or an undercoat on a metal, that can be chromium plated.

In the light of the foregoing description, the following is claimed:

1. A method of electrodepositing crack-free, chromium plate on an article of metal which comprises essentially: passing current in the range of 0.5 to 8 amperes per square inch from an anode to said article as a cathode immersed in an aqueous chromium plating bath at a temperature of 140 F. to the boiling point of the bath, said bath comprising essentially 200 to 900 g./l. CI'Os, strontium sulfate and an alkali metal silicofluoride each in an amount sufiicient to saturate said bath and to provide therein an undissolved residue of strontium sulfate and alkali metal silicofiuoride, respectively, said alkali metal being selected from the class consisting of potassium and sodium, a soluble non-catalytic strontium compound in an amount sufficient to adjust the concentration of the strontium sulfate in solution in said bath to 0.3 to 5.5 g./l. of sulfate (304 and a soluble non-catalytic alkali metal compound in an amount sufficient to adjust the concentration of the alkali metal silicofluoride in solution in said bath to 1.0 to 11.4 g./l. of silicofiuoride (SlFs the sum of said dissolved sulfate and silicofluoride being in the range of 1.5 to 11.7 g./l., said sum of dissolved sulfate and silicofiuoride varying with the C103 concentration in the manner defined by the area ABCD of the graph shown in the accompanying drawing, the alkali metal of said non-catalytic compound being the same as the alkali metal of said alkali metal silicofluoride.

2. A method according to claim 1 in which the metal article that is chromium plated is an article of plain carbon steel.

3. A method of electrodepositing crack-free,

chromium plate on a metal article which comprises essentially: passing current in the range of 1 to 3 amperes per square inch from'an anode to said article as a cathode immersed in an aqueous chromium plating bath at a temperature of 145 to 155 F., said bath comprising essentially 200 to 900 g./l. CI'Os, strontium sulfate and an alkali metal silicofluoride each in an amount sufiicient to saturate said bath and to provide therein an undissolved residue of strontium sulfate and alkali metal silicofluoride, respectively, said alkali metal being selected from the class consisting of potassium and sodium, a soluble non-catalytic strontium compound in an amount sufficient to suppress the concentration of the strontium sulfate in solution in said bath from the unsuppressed saturation concentration of the latter to a lower concentration equivalent to 0.3 to 4.5 g./l. of sulfate (S05), and a soluble non-catalytic alkali metal compound in an amount sufficient to suppress the concentration of the alkali metal silicofluoride in solution in said bath from the unsuppressed saturation concentration of thelatter to a lower concentration equivalent to 1.5 to 6.3 g./l. of silicofiuoride (SiFeF), the sum of said dissolved sulfate and silicofluoride being in, the range of 1.9 to 7.6 g./l., said sum of dissolved sulfate and silicofluoride varying with the CrOs concentration in the manner defined by the area JKLM of the graph shown in the accompanying drawing, the alkali metal of said non-catalytic compound being the same as the alkali metal of said alkali metal silicofiuoride.

4. A method according to claim 3 in which the alkali metal silicofiuoride is potassium silicofluoride and the alkali metal compound is a potassium compound.

5. A method according to claim 3 in which the alkali metal silicofluoride is sodium silicofluoride and the alkali metal compound is a sodium compound.

6. A method of electrodepositing crack-free, chromium plate on a metal article which comprises essentially: passing current in the range of 0.5 to 8 amperes per square inch from an anode to said article as a cathode immersed in an aqueous chromium plating bath at a temperature of to F., said bath comprising essentially 200 to 900 g./l. CrOa, strontium sulfate and an alkali metal silicoi'luoride each in an amount sufficient to saturate said bath and to provide therein an undissolved residue of strontium sulfate and alkali metal silicofluoride, respectively, said alkali metal being selected from the class consisting of potassium and sodium, a soluble non-catalytic strontium compound and a soluble non-catalytic alkali metal compound each in an amount sufficient to suppress the ooncentration of the strontium sulfate and alkali metal silicofluoride, respectively, in solution in said bath from the unsuppressed saturation concentration of the latter two compounds to a lower concentration equivalent to 1.5 to 11.7 g./l. of the sum of sulfate (394 and silicofiuoride (SiFs=), said sum of dissolved sulfate and silicofluoride varying with the CrOs concentration in the manner defined by the area ABOD of the graph shown in the accompanying drawing, the concentration of sulfate exceeding that of silicofluoride in the area EBF of said graph and the concentration of silicofiuoride exceeding that of sulfate in the area GHCD, the alkali metal of said non-catalytic compound being the same as the alkali metal of said alkali metal silicofluoride.

7. An improved method of producing a corrosion resistant chromium plated metal article cornprising plating the unplated article, in a single plating operation, in an aqueous bath comprising essentially 200 to 900 g./l. CrO3, strontium sulfate and an alkali metal silicofiuoride each in an amount sufficient to saturate said bath and to provide therein an undissolved residue of strontium sulfate and alkali metal silicofluoride, respectively, a soluble non-catalytic strontium compound and a soluble non-catalytic alkali metal compound each in an amount sufiicient to suppress the concentrations of the strontium sulfate and alkali metal silicofluoride, respectively, in solution in said bath from the unsuppressed saturation concentrations of the latter two compounds to lower concentrations equivalent to 0.3

23863 56 v 11 12 the range of 1.5m 11."7-g./1.,-sa,idsum 6f dissolved to 8 amperes per square -inch; and thereby'p'rosulfate and silicofiuoride varying with 'the-Croa ducing on the article a, crack-free deposit of concentration in the mannerdefined'bythe area, chromium. ABCD of the "graph shown in the accompanying drawing, the alkali metal of the non-catalytic References C ted in the fileof this patent alkaIi metal compound'andof the alkali metal U lT js'T T s PATENTS silicofluoride being the same and. being selected Numbe Name Date from the class conslstmg of potassmm and so- 2,640,022 stareck Qua-nuy 193.3

dium; performing the plating-at abath temperature of 140 to 180 F. and a current density'ofofi 10 

1. A METHOD OF ELECTRODEPOSITION CRACK-FREE, CHROMIUM PLATE ON AN ARTICLE OF METAL WHICH COMPRISES ESSENTIALLY: PASSING CURRENT IN THE RANGE OF 0.5 TO 8 AMPERES PER SQUARE INCH FROM AN ANODE TO SAID ARTICLE AS A CATHODE IMMERSED IN AN AQUEOUS CHROMIUM PLATING BATH AT A TEMPERATURE OF 140* F. TO THE BOILING POINT OF THE BATH, SAID BATH COMPRISING ESSENTIALLY 200 TO 900 G./L. CRO3, STRONTIUM SULFATE AND AN ALKALI METAL SILICOFLUORIDE EACH IN AN AMOUNT SUFFICIENT TO SATURATE SAID BATH AND TO PROVIDE THEREIN AN UNDISSOLVED RESIDUE OF STRONTIUM SULFATE AND ALKALI METAL SILICOFLUORIDE, RESPECTIVELY, SAID ALKALI METAL BEING SELECTED FROM THE CLASS CONSISTING OF POTASSIUM AND SODIUM, A SOLUBLE NON-CATALYTIC STRONTIUM COMPOUND IN AN AMOUNT SUFFICIENT TO ADJUST THE CONCENTRATION OF THE STRONTIUM SULFATE IN SOLUTION IN SAID BATH TO 0.3 TO 5.5 G./L. OF SULFATE (SO4=), AND A SOLUBLE NON-CATALYTIC ALKALI METAL COMPOUND IN AN AMOUNT SUFFICIENT TO ADJUST THE CONCENTRATION OF THE ALKALI METAL SILICOFLUORIDE IN SOLUTION IN SAID BATH TO 1.0 TO 11.4 G./L. OF SILICOFLUORIDE (SIF6=), THE SUM OF SAID DISSOLVED SULFATE AND SILICOFLUORIDE BEING IN THE RANGE OF 1.5 TO 11.7 G./L., SAID SUM OF DISSOLVED SULFATE AND SILICOFLUORIDE VARYING WITH THE CRO3 CONCENTRATION IN THE MANNER DEFINED BY THE AREA ABCD OF THE GRAPH SHOWN IN THE ACCOMPANYING DRAWING, THE ALKALI METAL OF SAID NON-CATALYTIC COMPOUND BEING THE SAME AS THE ALKALI METAL OF SAID ALKALI METAL SILICOFLUORIDE. 